Heterojunction tunneling field effect transistors, and methods for fabricating the same

ABSTRACT

The present invention relates to a heterojunction tunneling effect transistor (TFET), which comprises spaced apart source and drain regions with a channel region located therebetween and a gate stack located over the channel region. The drain region comprises a first semiconductor material and is doped with a first dopant species of a first conductivity type. The source region comprises a second, different semiconductor material and is doped with a second dopant species of a second, different conductivity type. The gate stack comprises at least a gate dielectric and a gate conductor. When the heterojunction TFET is an n-channel TFET, the drain region comprises n-doped silicon, while the source region comprises p-doped silicon germanium. When the heterojunction TFET is a p-channel TFET, the drain region comprises p-doped silicon, while the source region comprises n-doped silicon carbide.

RELATED APPLICATIONS

This application is a divisional application of U.S. Ser. No.11/307,331, filed Feb. 1, 2006.

FIELD OF THE INVENTION

The present invention relates to tunneling field effect transistors, andmethods for forming the same. More specifically, the present inventionrelates to heterojunction field effect transistors that comprise sourceand drain regions of different semiconductor materials for improvedtunneling current.

BACKGROUND OF THE INVENTION

As conventional complementary metal-oxide-semiconductor (CMOS) devicesare scaling down, the leakage current is increased, and the deviceperformance is degraded, due to increased short channel effects. Newdevices are needed to overcome these problems without requiring newmaterials and process steps.

The tunneling field effect transistor (TFET) is a quantum mechanicaldevice that shows reduced short channel effects and smaller leakagecurrents compared to standard complementary metal-oxide-semiconductor(CMOS) devices. FIG. 1 shows an exemplary n-channel TFET 10 that islocated in a p-well formed in a semiconductor substrate 12.Specifically, the n-channel TFET comprises a p-doped source region 22and an n-doped drain region 24, which are spaced apart from each otherby a channel region 23. A gate stack that comprises a gate dielectric 26and a gate conductor 28 is located over the channel region 23.

The structure of the TFET is based on a metal-oxide-semiconductor (MOS)gated pin-diode. In the off-state, the TFET works as a low-leakagereverse biased junction diode between the source and drain regions 22and 24. The barrier built by the reverse biased diode is larger comparedto the barrier of MOSFET. This results in reduced sub-threshold leakageand suppressed direct tunneling even at very short channel lengths. Whena bias is applied to the gate conductor 28, an electron channel isinduced in the channel region 23. Once the channel charge concentrationis degenerated, a tunneling junction is formed between the channelregion 23 and the source region 22, through which the tunneling currentflows.

As the TFET is scaling down from the 130 nm to the 90 nm node, use ofsteeper source doping gradients in the TFET has shown significantadvantages in improving the performance of the tunneling junction. Withthe steeper source doping gradients, the on-current of the 90 nm nodeTFET is no longer limited by the tunneling junction only, but by thecombination of the Zener diode and the MOS-channel. In the 65 nm nodeTFET, use of even steeper source doping gradients showed an on-currentonly limited by the MOS-channel. See Th. Nirschl et al., “The TunnelingField Effect Transistor (TFET) as an Add-on for Ultra-Low-Voltage Analogand Digital Processes,” IEDM (2004), pp. 195-198; see also Th. Nirschlet al., “The 65 nm Tunneling Field Effect Transistor (TFET) 0.68 μm² 6TMemory Cell and Multi-V_(th) Device,” Proceedings of ESSDERC (Grenoble,France 2005), pp. 173-176.

There is a continuing need for improved TFET structures with furtherenhanced tunneling currents.

SUMMARY OF THE INVENTION

The present invention seeks to further enhance the tunneling current ofa TFET device by providing a heterojunction at the source side of thechannel region of the TFET device. Such a heterojunction is formed byfabricating the source region of the TFET device using a semiconductormaterial that has a different band gap than the semiconductor materialused for forming the remaining components of the TFET device.

It has been discovered that the tunneling current of a TFET devicedepends not only on the abruptness of the source doping gradient, butalso on the band gap of the material used for forming the TFET device.Therefore, a heterojunction formed by a suitable semiconductor materialof a different band gap can be constructed and arranged to furtherenhance the junction current in the TFET device. For example, silicongermanium has a smaller band gap than silicon, and it can therefore beused to form the source region of an n-channel TFET device, therebyproviding an abrupt junction between the source and channel regions forfurther enhancing the tunneling current in the n-channel TFET device. Onthe other hand, silicon carbide can be used to form the source region ofa p-channel TFET device for further enhancing the tunneling current inthe p-channel TFET device.

In one aspect, the present invention relates to a tunneling field effecttransistor (TFET) comprising:

-   -   a. a drain region that comprises a first semiconductor material        of a first conductivity type;    -   b. a source region spaced apart from the drain region, wherein        the source region comprises a second, different semiconductor        material of a second, different conductivity type;    -   c. a channel region located between the source and drain        regions; and    -   d. a gate stack located over the channel region, wherein the        gate stack comprises at least a gate dielectric and a gate        conductor.

In another aspect, the present invention relates to a semiconductordevice comprising:

-   -   a. a semiconductor substrate,    -   b. a tunneling field effect transistor (TFET) comprising spaced        apart source and drain regions with a channel region located        therebetween and a gate stack located over the channel region,        wherein the source, drain, and channel regions of the TFET are        located in the semiconductor substrate, wherein the drain region        of the TFET comprises a first semiconductor material of a first        conductivity type, wherein the source region of the TFET        comprises a second, different semiconductor material of a        second, different conductivity type, and wherein the gate stack        of the TFET comprises at least a gate dielectric and a gate        conductor; and    -   c. a metal-oxide-semiconductor field effect transistor (MOSFET)        adjacent to the TFET, wherein the MOSFET comprises spaced apart        source and drain regions with a channel region located        therebetween and a gate stack located over the channel region,        and wherein the source, drain, and channel regions of the MOSFET        are located in the semiconductor substrate.

In a further aspect, the present invention relates to a method forforming a tunneling field effect transistor (TFET), comprising:

-   -   a. providing a semiconductor substrate that comprises a first        semiconductor material;    -   b. forming a gate stack over the semiconductor substrate,        wherein said gate stack comprises at least a gate dielectric and        a gate conductor;    -   c. doping the semiconductor substrate to form a first doped        region and a second doped region in the semiconductor substrate,        wherein the first and second doped regions both are doped with a        first dopant species of a first conductivity type and are spaced        apart from each other by the gate stack;    -   d. selectively etching the second doped region to form a recess        in the semiconductor substrate;    -   e. selectively forming a semiconductor structure in the recess,        wherein said semiconductor structure comprises a second,        different semiconductor material of a second, different        conductivity type,    -   f. thereby forming a TFET, which comprises a drain region formed        by the first doped region, a source region formed by the        semiconductor structure, and a channel region that is located        between and defined by the source and drain regions with the        gate stack located over the channel region.

Other aspects, features and advantages of the invention will be morefully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a conventional n-channel TFET.

FIG. 2 shows a cross-sectional view of an exemplary SiGe-basedheterojunction n-channel TFET, according to one embodiment of thepresent invention.

FIG. 3 shows a cross-sectional view of an exemplary SiC-basedheterojunction p-channel TFET, according to one embodiment of thepresent invention.

FIG. 4 shows the band graph of the SiGe-based heterojunction n-channelTFET of FIG. 2, in comparison with that of a conventional n-channelTFET.

FIG. 5 shows the band graph of the SiC-based heterojunction p-channelTFET of FIG. 3, in comparison with that of a conventional p-channelTFET.

FIGS. 6A-6F illustrate exemplary processing steps for fabricating aheterojunction TFET of the present invention.

DETAILED DESCRIPTIONS OF THE INVENTION, AND PREFERRED EMBODIMENTSTHEREOF

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide a thoroughunderstanding of the present invention. However, it will be appreciatedby one of ordinary skill in the art that the invention may be practicedwithout these specific details. In other instances, well-knownstructures or processing steps have not been described in detail inorder to avoid obscuring the invention.

It will be understood that when an element as a layer, region orsubstrate is referred to as being “on” or “over” another element, it canbe directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “beneath” or “under” another element, it can bedirectly beneath or under the other element, or intervening elements maybe present. In contrast, when an element is referred to as being“directly beneath” or “directly under” another element, there are nointervening elements present.

The present invention provides a heterojunction TFET structure forfurther enhancing the tunneling current in the TFET device.

Specifically, the drain region of the heterojunction TFET of the presentinvention comprises a first semiconductor material (e.g., Si, Ge, SiGe,SiC, or any other Group III-V semiconductor material) of a firstconductivity type (i.e., either n- or p-type conductivity). The sourceregion of the heterojunction TFET of the present invention, which isspaced apart from the drain region, comprises a second semiconductormaterial that is different from the first semiconductor material (e.g.,Si, Ge, SiGe, SiC, or any other Group III-V semiconductor material, butdifferent from the drain semiconductor), which has a second conductivitytype that is different from the first conductivity type (i.e., either p-or n-type conductivity). The channel region located between the sourceand drain regions also comprises the first semiconductor material havingthe first conductivity type, while the bandgap of the drain region issmaller than that of the channel region. In this manner, an abruptheterojunction (which refers to the junction between the source regionand the channel region) is formed between the channel region and thesource region of the TFET. Such an abrupt heterojunction functions tofurther enhance the tunneling current.

FIG. 2 shows a cross-sectional view of an exemplary heterojunctionn-channel TFET 30, according to one embodiment of the present invention.

The heterojunction n-channel TFET 30 is preferably fabricated on asemiconductor substrate 32, which may comprise any semiconductormaterial including, but not limited to: Si, Ge, SiGe, SiC, SiGeC, Ga,GaAs, InAs, InP and all other III/V or II/VI compound semiconductors.Semiconductor substrate 32 may also comprise an organic semiconductor ora layered semiconductor such as Si/SiGe, a silicon-on-insulator (SOI) ora SiGe-on-insulator (SGOI). Preferably, the semiconductor substrate 32is composed of a Si-containing semiconductor material, i.e., asemiconductor material that includes silicon. For example, thesemiconductor substrate 32 may consist essentially of silicon (e.g.,single crystalline, polycrystalline, amorphous, or a mixture thereof) ina bulk form. Alternatively, the semiconductor substrate 32 may comprisea SOI structure that contains a thin silicon layer located over a buriedinsulator layer, which functions to reduce the leakage current in thesubstrate.

The semiconductor substrate 32 may be doped, undoped or contain bothdoped and undoped regions therein. The semiconductor 32 may also includea first doped (n- or p-) region and a second doped (p- or n-) region.The first doped region and the second doped region may be the same, orthey may have different conductivities and/or doping concentrations.These doped regions are known as “wells” and can be used to definevarious device regions.

Further, the semiconductor substrate 32 can be a hybrid substrate, whichcomprising at least one region (e.g., the n-channel FET device region)in which mobility of electrons is enhanced, and another region (e.g.,the p-channel FET device region) in which mobility of holes is enhanced.By fabricating the n-channel FET in the electron-mobility-enhancedregion and the p-channel FET in the hole-mobility-enhanced region, themobility of the respective charge carriers (i.e., either electrons orholes) in the n-channel FET and p-channel FET device regions cansimultaneously be enhanced, thereby improving the CMOS deviceperformance.

More specifically, the semiconductor substrate 32 can be a hybridsubstrate that comprises different regions of different crystalorientations (which is referred to herein as a hybrid crystalorientation substrate). Functionality of such hybrid crystal orientationsubstrates is based on the anisotropy of carrier mobility in thesemiconductor crystals. Specifically, the mobility of charged carriessuch as electrons and holes varies with the crystal orientation of thesemiconductor substrate. For example, hole mobility is enhanced for a(110) surface in comparison to a (100) surface in silicon substrate, butelectron mobility is enhanced for the (100) silicon surface as comparedto the (110) surface. Therefore, by fabricating the n-channel FET in adevice region having the (100) surface crystal orientation, and thep-channel FET in a different device region having the (110) surfacecrystal orientation, the mobility of the respective charge carriers(i.e., either electrons or holes) in the n-channel FET and p-channel FETdevice regions are both enhanced. Such carrier mobility anisotropy alsoexists in other semiconductor materials, such as other group IVsemiconductor materials as well as group III-V and II-VI compounds, andthe hybrid crystal orientation technology (which is typically referredto as the HOT technology) therefore is readily applicable to substratescomposed of such other semiconductor materials. The hybrid crystalorientation substrate can be formed, for example, by a method thatincludes wafer bonding, selective etching and regrowth of asemiconductor layer, as described, for example, in U.S. patentapplications Ser. Nos. 10/250,241 and 10/932,982, the content of whichis incorporated herein by reference in its entirety for all purposes.

In a particularly preferred embodiment of the present invention, theheterojunction n-channel TFET 30 is fabricated on a p-well of thesemiconductor substrate 32, as shown in FIG. 2. The p-well functions toisolate the n-channel TFET 30 from other devices (not shown) located onthe semiconductor substrate 32 to the periphery of the TFET device 30.

Alternatively or additionally, at least one isolation region (not shown)can be provided in the semiconductor substrate 32 to isolate then-channel TFET 30. The isolation region may be a trench isolation regionor a field oxide isolation region. The trench isolation region is formedutilizing a conventional trench isolation process well known to thoseskilled in the art. For example, lithography, etching and filling of thetrench with a trench dielectric may be used in forming the trenchisolation region. Optionally, a liner may be formed in the trench priorto trench fill, a densification step may be performed after the trenchfill and a planarization process may follow the trench fill as well. Thefield oxide may be formed utilizing a so-called local oxidation ofsilicon process.

The n-channel TFET 30 comprises a source region 42, a drain region 44, achannel region 43 located between the source and drain regions 42 and44. A gate stack, which comprises a gate dielectric 46 and a gateconductor 48, is located over the channel region 43.

The drain region 44 of the n-channel TFET 30 comprises a firstsemiconductor material that is doped with an n-type dopant species. Thefirst semiconductor material can comprise Si, Ge, SiGe, SiC or any otherGroup III-V semiconductor material. The n-type dopant species can be anelement from Group IIIA of the Periodic Table of the Elements, such asnitrogen, phosphorous, arsenic, antimony, etc. The channel region 43also comprises the first semiconductor material and is doped with then-type dopant species.

The source region 42 of the n-channel TFET 30, on the other hand,comprises a second semiconductor material, which is different from thatof first semiconductor material. The second semiconductor material isselected from the group consisting of Si, Ge, SiGe, SiC or any otherGroup III-V semiconductor material. Further, the source region 42includes a p-type dopant species, which can be an element from Group VAof the Periodic Table of the Elements, such as aluminum, boron, gallium,indium, etc.

Preferably, the drain region 44 comprises silicon, and the source region42 comprises silicon germanium. Silicon germanium has a smaller band gapthan that of silicon. Therefore, by forming the source region 42 withsilicon germanium and the drain region 44 with silicon, a more abruptjunction with a smaller band gap is formed, as shown in FIG. 4 (thesolid line indicates the band graph of a SiGe p⁺ source, in comparisonwith the band graph of a Si p⁺ source indicated by the dashed line).Since the tunneling current I=A×exp^((−Eg/2KT)), wherein A is amultiplicity factor, and Eg is the band gap of the semiconductormaterial used for forming the source region, provision of such an abruptjunction with a smaller band gap functions to enhance the tunnelingcurrent.

Preferably, the germanium content in the source region 42 at thejunction immediately adjacent to the channel region 43 should not exceedthe critical germanium content allowed. The term “critical germaniumcontent allowed” is defined hereinbelow.

Pseudomorphically grown SiGe films contain intrinsic strain caused bylattice mismatch between the silicon substrate (having a smaller latticeconstant) and the SiGe film (having a larger lattice constant). When thestrain reaches a critical level, either due to increase of the SiGe filmthickness or increase of the Ge content, it can no longer be containedby elastic energy stored in the distorted SiGe crystal structure.Instead, a portion of the strain will be relaxed through generation ofmisfit dislocations in the heteroepitaxial interface. Therefore, for aSiGe film of a specific Ge content, there exists a “critical thickness,”defined as the maximum thickness for the pseudomorphic growth of theSiGe film, below which the strain caused by lattice mismatch between Siand Ge is contained by elastic energy stored in crystal latticedistortion, and above which a portion of the strain is relaxed throughgeneration of misfit dislocations in the heteroepitaxial interface.Similarly, for a SiGe film of a specific thickness, there exists a“critical Ge content,” which is defined as the maximum germanium contentthat can be incorporated into the pseudomorphic SiGe film below whichthe strain caused by lattice mismatch between Si and Ge is contained byelastic energy stored in crystal lattice distortion, and above which aportion of the strain is relaxed through generation of misfitdislocations in the heteroepitaxial interface. Dislocation defectsoriginated from strain relaxation are electrically active and can causeincreased carrier scattering, carrier trapping, and carrierrecombination. Therefore, it is preferred to maintain the Ge content andthe thickness of a SiGe-containing layer below the critical values, inorder to avoid formation of dislocation defects in the device structure.

Critical germanium content for a SiGe-containing layer can be readilycalculated by various conventionally known methods, as described by J.C. Bean et al., “Ge_(x)Si_(1-x)/Si Strained-Layer Superlattice Grown byMolecular Beam Epitaxy,” J. VAC. SCI. TECHNOL., Vol. A2, No. 2, pp.436-440 (1984); J. H. van der Merwe, “Crystal Interfaces. Part I.Semi-Infinite Crystals,” J. APPL. PHYS., Vol. 34, No. 1, pp. 117-122(1963); J. M. Matthews and A. E. Blakeslee, “Defects in EpitaxialMultilayers I. Misfit Dislocations in Layers,” J. CRYSTAL GROWTH, Vol.27, pp. 118-125 (1974); S. S. Iyer et al., “Heterojunction BipolarTransistors Using Si-Ge Alloys,” IEEE TRANSACTIONS ON ELECTRON DEVICES,Vol. 36, No. 10 (October 1989); R. H. M. van der Leur et al., “CriticalThickness for Pseudomorphic Growth of Si/Ge Alloys and Superlattice,” J.APPL. PHYS., Vol. 64, No. 5, pp. 3043-3050 (15 Sep. 1988); and D. C.Houghton et al., “Equilibrium Critical Thickness for Si_(1-x)Ge_(x)Strained Layers on (100) Si,” APPL. PHYS. LETT., Vol. 56, No. 5, pp.460-462 (29 Jan. 1990).

Typically, the critical germanium content is about 30 atomic %, but thespecific critical germanium content depends on the thickness of theSiGe-containing layer. Therefore, it is preferred that the germaniumcontent in the source region 42 at the junction immediately adjacent tothe channel region 43 is less than or equal to about 30 atomic %, andmore preferably ranges from about 10 atomic % to about 30 atomic %.

The SiGe-containing source region 42 of the present invention may have aflat Ge content profile (i.e., a substantially uniform Ge content isprovided across the entire source region 42), a multi-step Ge contentprofile (i.e., multiple plateaus of uniform Ge content are providedacross the entire source region 42), or a graded Ge content profile(i.e., the Ge content in the source region 42 gradually increases ordecreases), provided that the germanium content at the junctionimmediately adjacent to the channel region 43 is equal to or close tothe critical germanium content. Preferably, the source region 42 has agraded Ge content profile, which may have any suitable shape, eitherregular or irregular and which decreases along a direction away from thechannel region 43.

The gate dielectric 46 of the n-channel TFET 30 may comprise anysuitable gate dielectric material, such as semiconductor oxides,semiconductor oxynitrides, metal oxides, and combinations andmultilayers thereof. High k dielectric materials (with dielectricconstants greater than that of silicon dioxide) are particularlypreferred for forming the gate dielectric 46. More preferably, the gatedielectric 46 comprises a hafnium-based high k dielectric materialhaving a dielectric constant greater than about 10.0. Such hafnium-baseddielectric material can be selected from hafnium oxide (HfO₂), hafniumsilicate (HfSiO_(x)), hafnium silicon oxynitride (HSiON), a mixture ofhafnium oxide and zirconium oxide (ZrO₂), or multilayers thereof. Mostpreferably, the gate dielectric 46 of the n-FET gate stack compriseshafnium oxide or hafnium nitride.

The gate dielectric 46 can be formed by a thermal growing process suchas, for example, oxidation or oxynitridation. Alternatively, the gatedielectric 46 can be formed by a deposition process such as chemicalvapor deposition (CVD), plasma-assisted CVD, physical vapor deposition(PVD), metalorganic chemical vapor deposition (MOCVD), atomic layerdeposition (ALD), evaporation, reactive sputtering, chemical solutiondeposition, or any other suitable deposition processes.

The gate dielectric 46 may also be formed utilizing any combination ofthe above processes. The physical thickness of the gate dielectric 46may vary, but typically, the gate dielectric 46 has a thickness fromabout 0.5 to about 10 nm, with a thickness from about 0.5 to about 3 nmbeing more typical.

The gate conductor 48 may comprise any suitable conductive material,such as metal, metal alloy, metal nitride, metal silicide, or asemiconductor material such as Si or SiGe alloy in polycrystalline oramorphous form. The gate conductor 48 can be formed by any knowndeposition processes, such as, for example, CVD, PVD, ALD, evaporation,reactive sputtering, chemical solution deposition, etc. When the gateconductor 48 comprises a semiconductor material, such a semiconductormaterial is preferably doped by either an in situ doping depositionprocess, or by deposition followed by ion implantation and annealing.The ion implantation and annealing can occur prior to or after asubsequent etching step that patterns the material stack. The doping ofthe semiconductor material will shift the work function of the gateconductor 48 so formed.

The thickness, i.e., height, of the gate conductor 48 may vary dependingon the deposition process used. Typically, the gate conductor 48 has avertical thickness from about 20 to about 180 nm, with a thickness fromabout 40 to about 150 nm being more typical.

FIG. 3 shows a cross-sectional view of an exemplary heterojunctionp-channel TFET 50 formed in an n-well of a semiconductor substrate 52,according to one embodiment of the present invention.

The p-channel TFET 50 comprises a source region 62, a drain region 64, achannel region 63 located between the source and drain regions 62 and64. A gate stack, which comprises a gate dielectric 66 and a gateconductor 68, is located over the channel region 63.

The drain region 64 of the p-channel TFET 50 comprises a firstsemiconductor material and is doped with a p-type dopant species. Thefirst semiconductor material can comprise Si, Ge, SiGe, SiC or any otherGroup III-V semiconductor material. The p-type dopant species can be anelement from Group IIIA of the Periodic Table of the Elements, such asaluminum, boron, gallium, indium, etc. The channel region 63 alsocomprises the first semiconductor material and is doped with the p-typedopant species.

The source region 62 of the p-channel TFET 50, on the other hand,comprises a second semiconductor material, which is different from thatof first semiconductor material. The second semiconductor material isselected from the group consisting of Si, Ge, SiGe, SiC or any otherGroup III-V semiconductor material. Further, the source region 62 and isdoped with an n-type dopant species, which can be an element from GroupVA of the Periodic Table of the Elements, such as nitrogen, phosphorous,arsenic, antimony, etc.

Preferably, the drain region 64 comprises silicon, and the source region62 comprises silicon carbide. By forming the source region 62 withsilicon carbide and the drain region 64 with silicon, a more abruptjunction with a smaller band gap is formed, as shown in FIG. 5 (thesolid line indicates the band graph of a SiC n⁺ source, in comparisonwith the band graph of a Si n⁺ source indicated by the dashed line). Asmentioned hereinabove, since the tunneling current I=A×exp^((−Eg/2KT)),wherein A is a multiplicity factor, and Eg is the band gap of thesemiconductor material used for forming the source region, provision ofsuch an abrupt junction with a smaller band gap functions to enhance thetunneling current.

Preferably, the carbon content in the source region 62 at the junctionimmediately adjacent to the channel region 63 of the p-channel TFET 50does not exceed the critical carbon content allowed as definedhereinbelow.

Pseudomorphically grown SiC film contains intrinsic strain caused bylattice mismatch between the silicon substrate (having a larger latticeconstant) and the SiC film (having a smaller lattice constant). When thestrain reaches a critical level, either due to increase of the SiC filmthickness or increase of the C content, it can no longer be contained byelastic energy stored in the distorted SiC crystal structure. Instead, aportion of the strain will be relaxed through generation of misfitdislocations in the heteroepitaxial interface. Therefore, for a SiC filmof a specific C content, there exists a “critical thickness,” defined asthe maximum thickness for the pseudomorphic growth of the SiC film,below which the strain caused by lattice mismatch between Si and C iscontained by elastic energy stored in crystal lattice distortion, andabove which a portion of the strain is relaxed through generation ofmisfit dislocations in the heteroepitaxial interface. Similarly, for aSiC film of a specific thickness, there exists a “critical C content,”which is defined as the maximum carbon content that can be incorporatedinto the pseudomorphic SiC film, below which the strain caused bylattice mismatch between Si and C is contained by elastic energy storedin crystal lattice distortion, and above which a portion of the strainis relaxed through generation of misfit dislocations in theheteroepitaxial interface. Dislocation defects originated from strainrelaxation are electrically active and can cause increased carrierscattering, carrier trapping, and carrier recombination. Therefore, itis preferred to maintain the C content and the thickness of aSiC-containing layer below the critical values, in order to avoidformation of dislocation defects in the device structure.

Critical carbon content for a SiC-containing layer can be readilycalculated by various conventionally known methods. Typically, thecritical carbon content is about 5 atomic %, but the specific criticalcarbon content depends on the thickness of the SiC-containing layer.Therefore, it is preferred that carbon content in the source region 62at the junction immediately adjacent to the channel region 63 is lessthan or equal to about 5 atomic %, and more preferably ranges from about1 atomic % to about 5 atomic %.

The SiC-containing source region 62 of the present invention may have aflat C content profile (i.e., a substantially uniform C content isprovided across the entire source region 62), a multi-step C contentprofile (i.e., multiple plateaus of uniform C content are providedacross the entire source region 62), or a graded C content profile(i.e., the C content in the source region 62 gradually increases ordecreases), provided that the carbon content at the junction immediatelyadjacent to the channel region 63 is equal to, or close to, the criticalcarbon content. Preferably, the source region 62 has a graded C contentprofile, which may have any suitable shape, either regular or irregularand which decreases along a direction away from the channel region 63.

The heterojunction TFET of the present invention can be fabricated byusing a conventional CMOS process flow, with few or no additionalprocessing steps. Therefore, the heterojunction TFET of the presentinvention can be readily integrated side-by-side with MOSFET(s) to formimproved CMOS circuits. For example, a heterojunction n-channel TFET ofthe present invention can be integrated with a p-channel MOSFET, or aheterojunction p-channel TFET of the present invention can be integratedwith an n-channel MOSFET, to form an improved CMOS circuit.

FIGS. 6A-6F illustrate exemplary processing steps for fabricating aheterojunction TFET (either n-channel or p-channel) of the presentinvention.

Reference is first made to FIG. 6A, which shows a patterned gate stackthat comprises a gate dielectric 104 and a gate conductor 106 locatedover a semiconductor substrate 102. The patterned gate stack can bereadily formed by first depositing a blanket gate dielectric layer (notshown) and a blanket gate conductor layer (not shown), followed bypatterning of such blanket gate dielectric and gate conductor layersusing conventional lithographic techniques.

Next, an implantation step is carried out to implant the semiconductorsubstrate 102 with a first dopant species 1 (either n-type or p-typedopant). The patterned gate stack functions as a mask during suchimplantation step, thereby forming a first doped region 112 and a seconddoped region 114 that are both doped with the first dopant species 1, asshown in FIG. 6B. The first doped region 112 will function as the drainregion for the heterojunction TFET to be formed, and the region 113located underneath the patterned gate stack will function as the channelregion.

Subsequently, a dielectric masking layer 116, which comprises adielectric masking material such as silicon nitride, is deposited overthe entire structure, covering both the first and second doped regions112 and 114 and the patterned gate stack, as shown in FIG. 6C.

A portion of the dielectric masking layer 116 is then selectivelyremoved from the second doped region 114, to thereby exposed an uppersurface of the second doped region 114, as shown in FIG. 6D.

FIG. 6E shows selectively etching of the exposed second doped region114, using the dielectric masking layer 116 as a mask, to form a recess115 in the semiconductor substrate 102. The recess 115 preferably has adepth ranging from about 30 nm to about 150 nm.

A heteroepitaxial growth step is then carried out to grow asemiconductor structure 124 in the recess 115, as shown in FIG. 6F. Thesemiconductor structure 124 will function as the source region for theheterojunction TFET device to be formed, and it therefore comprises asemiconductor material that is different from that contained by thesemiconductor substrate 102. For example, when the semiconductorsubstrate 102 comprises silicon, the semiconductor substrate 124comprises SiGe or SiC. For formation of an n-channel TFET device, thesemiconductor structure 124 preferably comprises SiGe. For formation ofa p-channel TFET device, the semiconductor structure 124 preferablycomprises SiC.

The semiconductor structure 124 is doped by a second dopant species (notshown) that has a conductivity type opposite to the first dopant species1. The doping of the semiconductor structure 124 can be either carriedout via in situ doping during the epitaxial growth of the structure 124,or via dopant implantation after the epitaxial growth of the structure124. After formation of the doped semiconductor structure 124, thedielectric hard mask layer 116 can then be removed from the substratesurface using conventional stripping techniques.

Preferably, but not necessarily, the heterojunction TFET devices of thepresent invention are characterized by a channel length ranging fromabout 0.2 μm to about 10 μm and a channel width ranging from about 0.02μm to about 10 μm. More preferably, the channel length of theheterojunction TFET devices ranges from about 150 nm to about 250 nm,and the channel width ranges from about 0.5 μm to about 1.5 μm.

While FIGS. 2-3 and 6A-6F illustratively demonstrate several exemplarydevice structures and processing steps that can be used to form suchdevice structures, according to specific embodiments of the presentinvention, it is clear that a person ordinarily skilled in the art canreadily modify such device structures as well as the process steps foradaptation to specific application requirements, consistent with theabove descriptions. For example, while the semiconductor substratesshown in FIGS. 2-3 and 6A-6F represent bulk semiconductor substrates, itshould be appreciated that semiconductor-on-insulator (SOI) substratescan also be used for practice of the present application. For anotherexample, while the device structures shown in FIGS. 2-3 are designed foruse as field effect transistors in the CMOS technology, it is clear thata person ordinarily skilled in the art can readily modify the fieldeffect transistors of the present invention for use in otherapplications, such as diodes on SOI substrates, etc. It should thereforebe recognized that the present invention is not limited to the specificembodiment illustrated hereinabove, but rather extends in utility to anyother modification, variation, application, and embodiment, andaccordingly all such other modifications, variations, applications, andembodiments are to be regarded as being within the spirit and scope ofthe invention.

1. A method for forming a tunneling field effect transistor (TFET),comprising: providing a semiconductor substrate that comprises a firstsemiconductor material; forming a gate stack over the semiconductorsubstrate, wherein said gate stack comprises at least a gate dielectricand a gate conductor; doping the semiconductor substrate to form a firstdoped region and a second doped region in the semiconductor substrate,wherein the first and second doped regions both are doped with a firstdopant species of a first conductivity type and are spaced apart fromeach other by the gate stack; selectively etching the second dopedregion to form a recess in the semiconductor substrate; selectivelyforming a semiconductor structure in the recess, wherein saidsemiconductor structure comprises a second, different semiconductormaterial of a second, different conductivity type, thereby forming aTFET, which comprises a drain region formed by the first doped region, asource region formed by the semiconductor structure, and a channelregion that is located between and defined by the source and drainregions, with the gate stack located over the channel region.
 2. Themethod of claim 1, wherein the selective etching of the second dopedregion and the selective formation of the semiconductor structure arecarried out by using a dielectric mask that selectively covers at leastthe first doped region.
 3. The method of claim 2, wherein the dielectricmask is formed by first depositing a blanket mask layer over the firstand second doped regions and the gate stack, and then selectivelyremoving a portion of the blanket mask layer to expose the second dopedregion.
 4. The method of claim 1, wherein the first semiconductormaterial comprises silicon.
 5. The method of claim 4, wherein the firstdopant species is an n-type dopant species, wherein the second,different semiconductor material comprises silicon germanium including ap-type dopant species.
 6. The method of claim 5, wherein the sourceregion of the TFET has a germanium concentration ranging from about 20atomic % to about 30 atomic % at a junction immediately adjacent to thechannel region.
 7. The method of claim 5, wherein the first dopantspecies is a p-type dopant species, wherein the second, differentsemiconductor material comprises silicon carbide including an n-typedopant species.
 8. The method of claim 7, wherein the source region ofthe TFET has a carbon concentration ranging from about 2.5 atomic % toabout 5 atomic % at a junction immediately adjacent to the channelregion.